This invention relates to the production of extremely thin coatings of platinum on certain types of substrate. The resulting material may be used for many of the purposes for which platinum is used, including heterogenous catalysts for gas phase and liquid phase reactions and electrodes for electrochemical processes. This invention contemplates producing a thin platinum coating on a substrate with the result that the coated material has similar electrochemical properties to those of bulk platinum.
There are many techniques for coating platinum or other metal onto a substrate and perhaps the most well known is that which has come to be identified as electroplating or electrodepostion. This technique involves passing an electric current between two electrodes immersed in an electrolyte containing the coating metal. The substrate forms one of the electrodes and the passage of current causes the coating metal to deposit onto the surface of the substrate. This technique is highly desirable for many applications, but in the preparation of extremely thin, nonporous coatings it has limitations, particularly when the surface of the substrate is irregular as distinguished from flat and smooth. Other techniques such as thermal decomposition, vapor deposition, etc. suffer from one or more disadvantages when thin coatings are to be produced.
The present invention involves a procedure which does not include the application of an electric current from an external source. These procedures are sometimes known as electroless procedures and some developments have been made in this area in the prior art. In U.S. Pat. No. 3,342,628 to Sinclair a platinum-iron alloy is coated onto a ferrous metal substrat by dipping the ferrous substrate into a molten bath of calcium, calcium cloride, and platinum, and holding the substrate in that bath for a time sufficient to produce a coating of a platinum-iron alloy on the surface of the substrate. There is no indication that this process can produce a thin coating of platinum alone, as opposed to a coating of a platinum-iron alloy, or that it can produce such a coating on any substrate other than a ferrous metal substrate. In U.S. Pat. No. 3,461,058 to Haley et al there is described an electroless deposition or an electrodeposition or relatively thick, non-porous coatings (0.03-1 mil) of platinum followed by heat treatment and mechanical working. Such coating have little relationship to the extremely thin coatings of the present invention. In U.S. Pat. No. 3,630,768 to Bianchi there is described a procedure for depositing platinum onto a base metal using amino compounds in a chemical deposition method. Various platinum compounds are employed but there is no use of a molten salt as in the present invention, and the product of this patent contains relatively large amounts of platinum in the coating. In U.S. Pat. No. 3,635,761 to Haag there is described the electroless deposition of platinum onto any of several types of substrates employing either thermal decomposition of organometallic compounds or the use of hydrogen gas or other reducing agents. This procedure does not employ a molten salt bath as is specified in the present invention.
It is the object of this invention to produce extremely thin coatings of platinum on a substrate by an electroless process. It is another object of this invention to produce a platinum-coated substrate which exhibits similar electrochemical properties to those of bulk platinum. It is still another object of this invention to provide a process of coating a substrate with a thin layer of platinum from a molten salt bath.
The novel features believed to be characteristic of this invention are set forth with particularity in the appended claims. The invention, itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description.